the Ancients, will perhaps here find the Place to which they belong.

Of ALLOM, and the Minerals out of which it is extracted.

424. Allom (O) is a Salt of a white or a light reddish Colour, and of a sweet styptick Tafte. When diffolved in Water, and duly infpiffated and grown cold, it forms femi-tranfparent octoedral Crystals, which firft melt, boil, and foam in the Fire, and emit Phlegm in great Plenty, and then turn into a light, fpungy, and very white Mafs: But the Fire being increafed, they at laft give the fame acid Spirit as Vitriol (§ 417), or burning Sulphur (§ 312.) do, there remaining a light kind of white Earth, nearly of the Nature of (§ 31, 2.) Marl.

425. Allom (§ 424.) is produced, either out of a black, fhining, bituminous Mineral, easy to be cleft; hence of the Nature of (§ 319.) Sea-Coals, very uniform, but light, fometimes fibrous, and resembling Wood; or out of a foft bituminous Lump. Thefe Lumps are inflammable, and, when gathered into Heaps, they of themfelves grow hot, and take Fire, fending forth mean while a foetid, bituminous, ftrong Odour, and leaving after they are burnt a kind of Ipongy Earth, that has a little Tafte. But when they do not take Fire, but only grow warm, and cleave; then they become very fruitful of Allom: therefore, you must prevent their taking Fire, by pouring a little Water upon them; becaufe their happening to be inflamed renders the Maffes unfruitful.

426. But the other Minerals out of which Allom is produced, are not bituminous, nor of Courfe inflammable. To this Clafs belong fome Concretes, refembling common Clay, that cleave cafily as likewife the Calamine mentioned (§412). However the latter, as well as the foregoing (§ 425.) bituminous Species, requires to be gently roafted previously in an open Fire, to produce Allom.

427. It

427. It often happens, that, while the Pyrites is preparing for the Generation of Vitriol (§ 422.) a confiderable Quantity of Allom comes along together with the Vitriol; which may commonly be obferved in Ink-Stones (§ 421); as thefe often depofite Vitriol and Allom together, by pouring Water upon them; which however is not common to them all.

428. New, pure, aluminous Minerals, which are not of the Pyrite kind, when chemically examined, yield but very little of the Acid of Allom, or none at all, or at least not a Quantity answering to that of the Allom gotten out of them, after they have been a while expofed to the Air. Nor can one fee the Earth contained in thefe Minerals, which remains after the burning ($424.) of the Allom. The crude Calamine (§ 412, 423.) likewife when it is pure, not mixt with any Pyrites, nor accompanied with any fteel-grained Lead-Ore, appears to contain these Principles in a much lefs Quantity. Nor indeed is there any metallick or unmetallick Earth in this Stone, like the Caput Mortuum of (§ 424) Allom. But if we must believe that it contains Acid in reality; it is hardly conceivable, how it can lie fo much concealed therein. Now we leave it to the Judgment and Experience of every body, to determine whether the Acid of Allom, carried by the Air, is thus depofited in the Matrix of the Allom, according to the Opinion of the illuftrious F. Hoffman, or if the new Generation of the two Principles of Allom, that is of the terreftrial and the faline Acids ($424.) is made by Means of the Air, or of the Fire, or of both together, as is thought by Henckel; but, as it is hardly poffible to have an accurate Notion of this Matter, before we know the Operations, we shall have Occafion to treat of thefe Things more at large, when we come to our practical Part.

• Obferv. Phyfico-Chemic. L. 111. Obf. 8. pag. 275

Pyritologia, pag. 756.

Of

Of COMMON SALT, and the Methods by which it is obtained.

429. There are in feveral Countries Mines eafily come at, out of which the pureft, native, common Salt (§ 20.) is gotten in a folid Form: it is called Sal Gem, or Rock-Salt. This Salt is perfectly pure, white, half-tranfparent, and different from the other common Salt, by its large, compact, and apparently homogeneous Cryftals, which larger Crystals produced by Art, are a Collection of many smaller Ones: It is much more permanent in the common Air, nor does it contract a Moisture fo foon as the vulgar Kind of Salt. But these fo very pure Pieces of Sal Gem are not found in fo great Plenty; but, as one may easily guefs, they are now and then infected with other compound Minerals that mix with them. But, when these foreign Bodies are mixt with the Salt in large Quantities, it is freed of them, by diffolving it in Water, letting it fettle, and then ftraining it; and finally, by evaporating the Brine (for that is the Name of the Solution of it in Water), the Salt is again restored to its folid Form. But then it is no longer ftyled Sal Gem *.

430. The whole Ocean, and the feveral mediterranean Seas, (or larger Lakes) are full of common Salt: Nevertheless, the Waters of them all are not equally faturated with it. Some, in one Pound of Water, will have above an Ounce of Salt diffolved; and fome others hardly half an Ounce, and even less. Above all, the Seas that lie under and about the torrid Zone, are more full of Salt, than the Icy Seas towards the North. The Salt, which is fetched out of them, either by the bare Evaporation in the Sun

* But Sea-Salt, or the common Salt used in domeftick Affairs,

and

and the dry Air +, or by being infpiffated on a Kitchen Fire, is called Marine or Sea-Salt 1.

431. Finally, there are many Springs, the Water's of which are impregnated with common Salt, out of which it is obtained in the manner mentioned (§430): They call it Sal fontium.

432. These three Species of common Salt (§ 429431.) do not differ upon any other Account than 1. That of the Place where they are found, and from which they have all their peculiar Names. 2. Of their Purity; for the heterogeneous Bodies mixt with them, fpoil the Colour and outward Look of the Salt. This is the Reason why fome Brines drop their Salt eafily, and without any Addition, by bare Evaporation, and fome others with great Difficulty, or not at all, unless you add clarifying Ingredients. Mean while, as fuch foreign Bodies are not effential, but merely accidental to common Salt; they no way conftitute a real Difference; for any common Salt, provided it be well purified, appears always one and the fame , after many Examinations. You may read on thefe Subjects F. Hoffm. Obf. Phyf. Chemic. Lib. II. Obf. XVI. G. Agric, de Re Metallica, Lib. XI. P. Snellius de Sale communi, and others.

433. Salt Ammoniac may be referred to this Clafs, as being a femi-volatile kind of common Salt. They

Is called in English Bay-Salt, the finest of which comes from the Rocks on the Coafts of Sicily, and great Plenty of it is prepared in the Salt-Ponds near la Rochelle in France.

Great Quantities of which is made in England by diffolving Rock Salt in Sea-Water, and evaporating the Water till it bears an Egg, which being fet to cool, the Salt fhoots in it in the Form we have it for Ufe; the Cryftals being firft dried. The Brine which is poured off, is of a very bitter Tafte, and is therefore called Bittern; this again, boiled down, and fet to cryftalize, gives a bitter tafted Salt fo much in ufe of late for Purging, and is commonly fold under the Name of Etfom Salt, from its Reiemblance to the Salt prepared out of those medicinal Waters.

Bay-Salt is the most impure, having more earthy Particles in it. Concerning the best Method of making Salt or Sea-Water fresh and wholesome to drink, fee those curious Experiments in a Book entituled Philofophical Experiments, &c. by Stephen Hales, D. D. F. R. S. Lond. 1739, in 8°.

fay, that this foffil Salt is found native about the Vulcano's, or Mountains that vomit Fire, as alfo in the fandy, dry, burning Places that are about and under the torrid Zone; and that it is likewife evaporated from the Crevices of the Rocks in Perfia. But the vulgar Salt Ammoniac which is fold, is all artificial, and is produced out of the Bodies, which actually, and in reality contain common Salt, or its Acid, and the volatile alkaline Salt. Because these are two Principles, of which every Kind of Salt Ammoniac confifts, and into which it may be refolved by Art; therefore the Urine of Animals, especially of thofe that ufe common Salt; the Soot of many inflammable Bodies, &c. yield this Salt. They fay that the Salt which is brought us from Egypt, which is the most common, is prepared by Sublimation, either out of the Soot of burnt Dung* of Animals alone, or out of this very Soot mixt again with common Salt, and with the Urine of Animals. See Alla Parifina, and Boerhaave's Elem. Chem. T. II. Proc. cII. That which is brought from the Eaft-Indies, not in flat convex Loaves, but in conical Ones, like fugarLoaves, is rare, and the Manner in which it is made, unknown. It is enough for us to be certain, that every true Salt Ammoniac may be refolved into the two Principia before mentioned, and that likewife out of thefe, Salt may be made in every Refpect perfectly like native or artificial Salt.

Of BORACE, and its Origine.

434. We have already (§ 131.) treated of Borace, as of a Menftruum. It is otherwife, though lefs usually, called Chryfocolla, that is, Gold-Sodder: tho' at the fame Time, this Name is much properer to it, than to the green Oker (§ 366.) of Copper; which, though called by the fame Name, yet is altogether

Especially the Dung of Pidgeons, and the Urine of Camels.

different